An intrepid group of astronauts: Reed, Sue, Johnny and Ben, originally received their powers after being exposed to an immense amount of cosmic radiation during a trip into space. Mutated and given strange powers by this exposure, they became the Fantastic Four and dedicated the use of their powers for the benefit of humanity.

Well, The Thing was briefly a member of the Avengers in the 80s and came back for the New Avengers whereas Mr. Fantastic and Invisible Woman were Avengers for 5 or so issues when they weren't on the Fantastic Four after Inferno.

@Fuchsia_Nightingale: In the 80s, Thing was briefly a member of the Avengers but he left the issue after properly joining because he was being mopey. Then when Marvel did their last big relaunch about a year ago he become an Avenger again.

And Mr Fantastic and Invisible Woman were Avengers from #299 until #303 (i think it was)

The FF have been honorary/reserve members of Avengers for a long time. They just never officially joined until the times stated above. It's the same thing as how Spider-man always seems to show up in FF comics but was never a real everyday member until.